To contain electricity, you need insulators. Before electricity was
made, there was the Big Electricity to deal with: lightning! So,
the earliest glass insulators were LRIs (Lightning Rod Insulators),
used to insulate lightning rods and cables from structures. They date
to the 1840s or earlier, and were made in glass until about the turn of
the century.

Lightning Rod Insulators

Battery Rest Insulators

In 1844, Morse's first telegraph line was installed between Baltimore
and Washington. To produce the current needed, wet cell DC batteries
were used: glass tanks full of electrolytic solution with copper, carbon,
or zinc electrodes. These batteries were often insulated from their
supports with "battery rest insulators".
See Richard Dawson's display for more.

The early years of telegraph development saw rapid change in insulator
design: it wasn't yet known what worked well, or why. Many designs
were fine when first installed (usually during mild, dry weather), but
failed quickly in the wet season, or became damaged due to big temperature
swings. "Ramshorn" patterns (which held the wire suspended beneath) were
the most popular, and were made through the 1870s.

3 Early Telegraph Insulators

Threadless Insulators

Eventually one general design proved superior: an inverted cup shape with
a groove where the line wire is attached with a tie wire, and placed on a
wood "pin": a "pin-type" insulator. The insulator was glued to the
pin with molten sulphur, tar, burlap, etc, and the pin set in a hole in
a crossarm. Since the pin-hole was plain and smooth, these are called
"threadless".

The problem was the glueing: it's very difficult to get something to stick
to smooth glass. The insulators would work loose, and in cold weather when
the line wire shrank, would pop off the pins. In 1865 Louis Cauvet patented
the idea of threading both the pin-hole and pin, thus securing the insulator.
This was the last major design change: insulators stayed about the same until
their production ended in the 1970s.